prop to VCR to TV

Roger Lee

I've been running prop video output into a VCR, then it's output to TV.
My thought is better safe than sorry.
This is not a hobby TV.
a current post by Rayman triggered this question.

Does this actually offer me any protection for the TV?

Roger

StefanL38

Hello Roger,

a composite-video-input is always coupled over a capacitor. So DC-voltages can't reach the VCR or TV anyway.
The VCR may offer some protection for the TV. If it should happen that you connect AC-Power from the wall-plug (120-240V) to the video-composite-plug
I think both VCR and TV will be fried. If you would like to have protection against this you could use a wireless tranceiver
but this tranceiver has to have a compostite-video input.

best regards

Stefan

Luis Digital

I think he meant that he is using radio frequency output of the VCR (channel 2 or 3).

Is this?

"a composite-video-input is always coupled over a capacitor"

Usually used optocouplers.

Post Edited (Luis Digital) : 11/4/2009 10:22:24 PM GMT

Mike Green

What are you trying to protect against?

Roger Lee

Q: What are you trying to protect against?

A: Damage to the TV.

It just seems somehow safer to run the video signal through
another device before the TV. I don't know, so I ask.

localroger

No. If you manage to program the Prop to output some weird scan frequency the TV can't match, it simply won't sync (unlike some VGA monitors of a certain era which could be damaged by carefully chosen out of range sync frequencies). And the signal is capacitive coupled, so unless you are powering your prop from a Tesla coil there is no way for it to put enough energy into the input to damage it. There is no likely path either by signal shape or voltage for the prop to damage a TV.

Mike Green

It doesn't make a lot of sense. The signal levels we're talking about are on the order of 1V, maybe 1.5V with very limited current available. We're talking about current on the order of 1-4mA. None of that is capable of damaging a TV's input circuitry.

The kinds of damage that might occur have nothing to do with a Prop. Lightning might strike and be conducted through connecting wiring no matter what else is in-line. You might accidentally drop a plugged in broken toaster on your Prop board which could accidentally connect the AC line to your TV's input circuitry, let alone fry your Prop. You could trip over the connecting cable which could break the socket on the TV.

Roger Lee

Thanks guys, good stuff to know.

potatohead

There is one potential damage path.

Really old TV's have poorly engineered horizontal fly back transformers, coupled with rather dumb circuits. They also have accumulated dust by now. Extreme signal changes that occur rapidly can cause a path to form and short the fly back. Somebody here can explain why maybe...

A few things like the full white screen, flashing to black, lose sync, go white, excessive vertical frequency, lower vertical frequency, etc... can trigger this, if the TV sees them in combination and rapidly over some period of time.

I had a really old one early on when writing video code for the prop, and sent it a few things that caused a distinct *Pop* and light hiss. (almost ready to arc) A can of compressed air later, all was good. This TV was a throw away, I scooped off the street as a quick 'n dirty prop monitor. Was a early 70's era model, and the dust was huge. Frankly, a very aggressive and valid signal may have cause that geezer some grief.

As a kid, I hammered one or two of them with 8 bit computers back then. Same deal. Throw ways, I fixed up enough to use for displays. That was the 80's and the TV's were kind of old then, so maybe that gives you an idea of the range I'm talking about.

Almost no television in use today is subject to this, and no TV manufactured now is, so I would not worry at all.

Any modern set has plenty of both over engineering (because the core circuits will work in multiple regions, NTSC, PAL, etc..), and much improved fly backs, or the complete lack of them in the case of newer LCD types. Only really old CRT TV's are weak in this regard, and usually only if they are poorly maintained and likely to die off anyway. 50's, 60's and 70's TV's are where the poor ones can be found. Not all from that age are bad, but the older you go, the more there are.

A VCR will protect somewhat as most VCRs made today will not pass through a poor signal. FYI: Older VCRs will absolutely pass it through, giving no effective protection. (early 80's battle tank style)

I wouldn't bother with one though, because that same protection inhibits you from seeing "almost" signals, and poorly structured signals. These will display and not harm any device, but may be corrected by the VCR, meaning you can't see the real effect of your code! Better to see it in the "raw"

If you can, get a nice, cheap USB capture card for the very best TV environment. Capture cards can be had for as little as $10, and render to a window on a PC, allowing for snapshots of the image, recording of movies to catch bugs, and just generally display nice. Most importantly, they don't take a lot of room!

Capture cards will display formats from around the world, PAL, NTSC, etc... and will generally display pretty whacked signals too. Good for zeroing in on a good display. Once it's looks great, display it on a few real TV's for a final vetting.

I totally understand wanting to see it "on the TV" though. There is nothing better, if you've got the room and time.

Bottom line, don't worry about your TV. I just thought I would post the info on older sets in the interest of being complete, not to warn you. Hook the prop right up to the tube and enjoy! It's fun, and my favorite, when I can do it.

[noparse]:)[/noparse]

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Post Edited (potatohead) : 11/5/2009 4:05:34 AM GMT